Recently, interests have been concentrated on sensors which can detect and analyze a target material using nano-materials in various application fields. In particular, portable nano-sensor systems for the rapid detection of specific odorants play important roles in anti-bioterrorism, disease diagnostics, and food safety.
Since the first report of “electronic nose” in relation to this, there have been many efforts to develop artificial olfactory sensors based on the arrays of semiconductor devices. “Electronic nose” refers to a system which may reproduce a human nose using sensor arrays and pattern recognition systems. However, the capabilities of the electronic noses thus developed are still inferior to those of the human olfactory system in terms of specificity.
In the smelling process of the human olfactory system, the initial step is to bind specific odorants to the olfactory receptor protein which triggers signal transduction in a cell. Olfactory receptors expressed in the cell membranes of olfactory receptor neurons are responsible for the detection of odorant molecules. That is, when the odorants bind to the olfactory receptors as described above, the receptors are activated. The activated olfactory receptors are the initial player in a signal transduction cascade which ultimately produces a nerve impulse which is transmitted to the brain. These olfactory receptors are members of the class A rhodopsin-like family of G protein-coupled receptors (GPCRs).
Over the last decade, the development of bioelectronic noses with olfactory receptors as a sensing part has been a major concern. In addition, various technologies such as quartz-crystal microbalance, electrochemical impedance spectrometry, surface plasmon resonance, and light-addressable potentiometric sensor have been used as a transducer part of the bioelectronic nose. However, these technologies have problems such as big system size and limited sensitivity.
Various transistors based on nano-structures, particularly single-walled carbon nanotube-field effect transistors (swCNT-FETs) have been widely studied in the high-selectivity biosensor, but swCNT-FETs have not been used in the application of the bioelectronic nose. This is partly because olfactory receptors are G protein-coupled receptors (GPCRs) which may maintain their functionality only as a part of the cell membranes, as described above. Thus, unlike other receptor molecules which may exist alone and be easily connected with the surface of the transducer, lipid membranes containing functional olfactory receptors should be formed on swCNT-FETs to build a bioelectronic nose, which is still a very difficult task.